The production of biodiesel occurs through the reaction of vegetable oils or animal fats with an alcohol in the presence of a catalyst to yield methyl esters (biodiesel) and glycerol. It's estimated that for every 9 kg of biodiesel produced, about 1 kg of by-product glycerol is formed. Thus, as biodiesel production increases, it's expected that large quantities of glycerol will become more readily available. It has therefore become desirable to develop effective refinement processes to convert such glycerol to value added commodity and specialty products, such as diols and other polyols, ketones, aldehydes, carboxylic acids and alcohols.
In particular, one such product is propylene glycol which is a widely used chemical in many industrial and consumer products. Traditionally produced by the hydration of petroleum-derived propylene oxide, various processes have recently been proposed for producing propylene glycol from glycerol. For example, U.S. Pat. No. 5,426,249 describes a process for simultaneously producing 1,2- and 1,3-propanediol by dehydrating gaseous glycerol to acrolein, hydrating the acrolein to 3-hydroxypropionaldehyde and subsequently hydrogenating the 3-hydroxypropionaldehyde in the presence of a conventional hydrogenation catalyst. U.S. Pat. Nos. 5,214,219 and 5,616,817 describe processes in which glycerol is hydrogenated to 1,2-propanediol using a copper/zinc or copper/cobalt catalyst at elevated temperatures and pressure. Finally, WO 05/095536 describes a process for converting glycerol to propylene glycol via an acetol intermediate at temperatures between about 150°-250° C. and a pressure of about 1-25 bar.
One drawback to the current processes is the formation of impurities and by-products during conversion that can cause undesirable color as well as odor in the propylene glycol product.
Thus, it is an object of the present invention to provide a process of reducing color and/or odor from the converted propylene glycol by essentially removing impurities and by-products produced during the conversion process. In addition, it is also an object of the present invention to provide an improved process for producing propylene glycol from glycerol at high selectivity and conversion and reduced operating costs.